The 425 seriously needs a grid over current sense circuit. The only problem
is that the way it's wired/designed, you can't break the PTT line when
transmitting and have it unkey... The RF sense circuit won't let it.
73
Jim W7RY
----- Original Message -----
From: Jim Brown
To: amps@contesting.com
Sent: Friday, January 25, 2008 7:45 AM
Subject: Re: [Amps] Ten Tec 425 Titan Partial Schematic
On Fri, 25 Jan 2008 06:14:18 -0500, David J Windisch wrote:
>What are your frequent maintenance issues?
Anything that creates a voltage spike or arc or short is likely to
take one or more small signal transistors on the Input Bias Board.
On two occasions, I've had to replace a half dozen of them at a
time! The voltage rating of those transistors is FAR too close to
the operating voltage.
I operating 90% CW, 100% QSK, and a lot of contesting. I'm an
agressive contester. That takes a toll in vacuum relays.
There's a trace on the rectifier board near the B- terminal (noted
on the schematic) that tends to fry under certain arc/short faults
(I've had it happen to me once, and seen another board that was
patched to repair the same thing. The time it happened to me blew
both mains fuses on turn-on.)
This issue with spikes, shorts, and arcing can be anything from
something coming in on the power line to an antenna problem
reflected to the amp to an arc over in a tube to a vacuum relay
failure.
Right now, two of my three amps are down. One probably has a bad output
tube (a pair of new 3CPX800A7 pulls with fewer than 100 hours, but a
few nasty jolts from antenna failures while transmitting). I pulled
the other one out of service because the keying is "stuttering,"
probably due to a bad relay. So I traded boards with another amp,
plugged it back in, it worked, but the dial lights were out. Opened
it back up, but didn't wait long enough for the HV to discharge (30
seconds or more), and the safety interlock arced. Changed the bulbs,
closed it back up, and now the grid meter is pegged (and the LED
lit) when I turn it on (and it isn't in operate mode yet -- the
filament timer is still going).
So it's back to the bench for two of these puppies when I have time.
Another issue is the HV filter caps -- many of the 425 amps are 20
years old. It is NOT necessary to replace them all when they fail, but
it's not uncommon for there to be a bad one in the string. You'll get
reports of a buzzy signal and see lower than normal plate voltage under
load.
73,
Jim K9YC
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